Scientists now have definitive proof that many of the landscapes seen on Mars were indeed cut by flowing water.
The valleys, channels and deltas viewed from orbit have long been thought to be the work of water erosion, but it is Nasa’s latest rover, Curiosity, that has provided the “ground truth“.
Researchers report its observations of rounded pebbles on the floor of the Red Planet’s 150km-wide Gale Crater.
Their smooth appearance is identical to gravels found in rivers on Earth.
Rock fragments that bounce along the bottom of a stream of water will have their edges knocked off, and when these pebbles finally come to rest they will often align in a characteristic overlapping fashion.
Curiosity has pictured these features in a number of rock outcrops at the base of Gale Crater. (BBC News)